Cardiac rhythm management (CRM) devices can help assist heart function, such as by providing pacing electrostimulations to evoke responsive heart contractions, cardiac resynchronization therapy (CRT) electrostimulations to coordinate the spatial nature of a heart contraction of one or more heart chambers, antitachyarrhythmia pacing, cardioversion, or defibrillation shocks to interrupt a tachyarrhythmia. In order to “capture” heart tissue near an electrode delivering electrostimulation energy, evoking a responsive depolarization and heart contraction, the electrostimulation energy must exceed a threshold value, sometimes referred to as a capture threshold. After determining the capture threshold, electrostimulations can be delivered in excess of the capture threshold to capture the heart tissue—too much electrostimulation energy may not be the best for the heart, moreover, it can waste energy and shorten the useful life of the device.
Sathaye et al. U.S. Patent Publication No. 2008/0046019, filed Aug. 17, 2006, and published Feb. 21, 2008 discloses a CRM device that can be pectorally-implanted and coupled to the heart via intravascular leads (sometimes referred to as catheters). In an example, a first lead can extend from the superior vena cava (SVC) into the right atrium (RA), and then into the right ventricle (RV). It can include an RV apical tip electrode, a slightly more proximal RV ring electrode, a still slightly more proximal RV shock coil electrode, and an even more proximal RA or SVC shock coil electrode. In an example, a second lead can extend from the SVC into the RA, through a coronary sinus (CS) into the coronary vasculature, such as near a portion of a left ventricle (LV). In an example, this second CS/LV lead can include first and second electrodes, from which electrostimulation energies can be delivered.
Determining an electrostimulation capture threshold can involve varying the electrostimulation energy up or down until the onset or loss of capture is detected. Whether capture occurs can be determined by observing the “evoked response” intrinsic heart signal waveform, after delivering electrostimulation, in the region of the electrode where the electrostimulation is delivered, such as described in Sathaye et al., U.S. Patent Publication No. 2008/0071319, filed Sep. 14, 2006, and published on Mar. 20, 2008 (see, e.g., FIGS. 2A, 2B, 3A, 3B and accompanying description). In sensing the evoked response signal, it is helpful to use one or more different electrodes than the electrodes from which the electrostimulation energy was delivered. This is because delivering an electrostimulation can cause a “pacing artifact” that can mask the desired evoked response signal if the same electrodes are used for both delivering the electrostimulation and sensing the evoked response. This can be particularly problematic, for example, for a CS/LV lead that includes only two electrodes for delivering a “dedicated bipolar” electrostimulation between these two electrodes located in the coronary vasculature in the LV region for evoking a responsive LV heart contraction. If there are only two CS/LV electrodes for delivering the electrostimulation, such as in a dedicated bipolar configuration, the resulting significant pace artifact can complicate a local measure of the resulting evoked response. This can preclude determining a capture threshold under conditions of dedicated bipolar electrostimulation.
Sathaye et al. U.S. Patent Publication No. 2008/0046019 discloses an example of how to estimate a “dedicated bipolar” capture threshold between two CS/LV electrodes by measuring an “extended bipolar” capture threshold between an LV electrode and an RV electrode, and using this measured information to estimate the “dedicated bipolar” capture threshold.